1. Field of the Invention
The present invention relates to an apparatus for combining a character font in an image subjected to data compression by encoding and stored in a recording medium.
2. Description of the Related Art
Image data prepared by converting an image obtained by a television camera etc. to a signal which can be treated by a digital signal processing apparatus such as a computer usually comprises a tremendous amount of information. Accordingly, if this image data is recorded in a recording medium as it is or is transmitted by modem by computer or the like, the efficiency would be very bad. Therefore, conventionally, information of image data has been compressed by encoding and then recorded in a recording medium or transmitted.
This data compression is carried out as follows.
First, the image data is subjected to analog-to-digital (A/D) conversion and stored in an image memory as digital image data. On the other hand, where a character font is superimposed at a predetermined pixel position of this image, the data of this character font is read out from a character font memory and the data of the image memory at that pixel position is substituted by the character font data for each pixel. In this way, the image data on which the character font is superimposed is read out from the image memory for every block of size consisting of 8.times.8 pixels, subjected to a discrete cosine conversion (hereinafter referred to as a DCT conversion), and converted to a DCT coefficient. This DCT coefficient is quantized, and this quantized DCT coefficient is subjected to Huffman encoding and data compression and then recorded in the recording medium.
Note that, Huffman encoding is an encoding procedure in which short code words are assigned to symbols having a high generation probability based on statistical analysis of the generation frequency of symbols of an information source and, at the same time, long code words are assigned to symbols having a low generation probability, whereby the average word length of the code words is shortened.
The image data (coded data) recorded in the recording medium is subjected to Huffman decoding and converted to the quantized DC coefficient and further subjected to inverse quantization to become a DCT coefficient. An inverse DCT conversion is carried out with respect to this inverse quantization DCT coefficient for every block consisting of 8.times.8 pixels so as to restore the image data. This image data is once written in the image memory, read out from this image memory, and converted to an analog signal and output to a display device.
Where a new character font is added to image data (coded data) recorded in the recording medium or where a character font which has already been superimposed on the image data is substituted by a new character font, after the original image data is restored from the encoded data, processing for adding the new character data to this restored data must be carried out. Namely, the encoded data read out from the recording medium is subjected to processings such as Huffman decoding, inverse quantization, inverse DCT conversion, etc., and then once written in this image memory, and the character font is written in the image data written in this image memory. Then, this image data is subjected to the processings of DCT conversion, quantization, Huffman encoding, etc. and recorded in the recording medium.
However, this type of decoding and encoding processing requires a long time, and enormous power is consumed by a circuit performing these processings.